The spontaneous noncopulator male rat presents a model useful for studying the mechanisms underlying sexual refractoriness in mammals. Any group of rats bred or purchased will contain a significant proportion of males which refuse to copulate. The present series of studies proposes to explore some physiological and experiential sources of such refractoriness. We also hope to explore concrete techniques for overcoming the problem. The physiological experiments include implantation of testosterone into the preoptic/anterior hypothalamic area of the brain, and creation of electrolytic lesions in the posterior hypothalamus and other limbic system sites. Psychological experiments include extended social exposure of noncopulators to estrous females, operant conditioning of sexual behavior, the effects of early handling on proportion of litter becoming sexually refractory, and some pilot studies with drugs. These experiments should narrow the range of hypotheses which can be proposed to account for the behavior of the noncopulators. Such insights can hopefully be applied to similar instances in other species where psychogenic infertility is of especial concern to man.